1. Field of the Invention
The present invention relates to a liquid crystal drive unit, a liquid crystal driving method, and a liquid crystal display device. More particularly, this invention is concerned with a liquid crystal drive unit, a liquid crystal driving method, and a liquid crystal display device capable of successfully achieving gray-scale display without deterioration in contrast or occurrence of flickers or crosstalk.
2. Related Art
Liquid crystal display devices have been applied to various fields in recent years. Examples of the application fields include the fields of relatively inexpensive information apparatuses such as personal digital assistants (PDA) and portable telephones, of portable game machines, and of home electric appliances. Many pieces of equipment having the liquid crystal display devices are intended to be portable. Along with a request for an apparatus usable for a long period of time without the necessity of recharge, a liquid crystal display device requiring low power consumption is on demand.
Conventional methods for realizing gray-scale display include a frame rate control method (FRC), a pulse width modulation method (PWM), and a pulse height modulation method (PHM). The frame rate control method is such that a plurality of frames of gray-scale data, which has been thinned, is used to display data with a gray scale. The pulse width modulation method is such that gray-scale data is weighted according to the widths of selection periods in order to achieve gray-scale display. The pulse height modulation method is such that gray-scale data is weighted according to applied voltages in order to achieve gray-scale display.
Among these methods, the FRC and PHM methods are implemented in display systems using an active matrix type liquid crystal display panel for display, and can achieve gray-scale display successfully. However, the circuitry of a signal line driver is likely to be complex and to be scaled up.
In contrast, the PWM method is utilized as one of gray-scale display methods to be implemented mainly in display systems having a passive matrix type liquid crystal display panel used for display. The gray-scale display method of the PWM method is adopted together with a method for achieving liquid crystal display according to a multi-line selection driving method (MLS, Japanese Unexamined Patent Publication No. 9-281463) to be described later, whereby effective gray-scale display can be achieved with excellent contrast ensured.
The conventional PWM gray-scale method based on the MLS driving method will be described in conjunction with the drawings.
FIG. 6 shows driving waves employed in displaying gray-scale display data shown in FIG. 9 using a four-level gray scale according to the PWM method. FIG. 10 shows the results of the arithmetic operations for MLS performed on gray-scale display data shown in FIG. 9 in relation to fields. Referring to FIG. 6, one horizontal period is divided in the ratio of 1:2. Assume that the short period is a period F and the long period is a period S. SEG4m+1 denotes a driving wave used to express gray-scale level 0, SEG4m+2 denotes a driving wave used to express gray-scale level 2, SEG4m+3 denotes a driving wave used to express gray-scale level 1, and SEG4m+4 denotes a driving wave used to express gray-scale level 3. SEG4m+1 that is used to express gray-scale level 0 represents a liquid crystal driving potential V2, which results from the arithmetic operations for MLS performed on the high-order and low-order display data items, during both the selection periods F and S. Likewise, SEG4m+4 that is used to express gray-scale level 3 represents a liquid crystal driving potential xe2x88x92V2, which results from the arithmetic operations for MLS performed on the high-order and low-order display data items, during both the selection periods F and S. SEG4m+2 that is used to express gray-scale level 2 represents the potential xe2x88x92V2, which results from the arithmetic operations for MLS performed on the low-order display data, during the selection period F. The SEG4m+2 represents the potential V2, which results from the arithmetic operations for MLS performed on the high-order display data, during the selection period S. SEG4m+2 that is used to express gray-scale level 1 represents the potential V2, results from the arithmetic operations for MLS performed on the low-order display data, during the selection period F. The SEG4m+3 represents the potential xe2x88x92V2, which results from the arithmetic operations for MLS performed on the high-order display data, during the selection period S.
However, the conventional PWM method has a problem with deterioration in display definition stemming from crosstalk or the like. The crosstalk is derived from differences among the frequency components of signal line driving waves associated with gray-scale levels, smoothing of signal electrode waves, or influence of signal electrode signals on the potentials at other liquid crystal elements via a scanning electrode. In short, the cause of the crosstalk is mainly a change in an applied voltage derived from the fact that the frequency of a driving voltage wave differs with a display pattern and that the drive voltage wave distorts at transparent electrodes.
In consideration of the foregoing problem, an object of an embodiment of the present invention is to provide a PWM type liquid crystal drive and driving method. Herein, the way of handling display data used to achieve gray-scale display and the gray-scale display control sequence are improved in order to realize low power consumption and a simpler configuration.
Another object of the embodiment of the present invention is to provide a liquid crystal display device having a relatively simple configuration, requiring low power consumption, and capable of successfully achieving gray-scale display without deterioration in contrast and occurrence of flickers or crosstalk.
The present inventors had a profound discussion in efforts to accomplish the above objects. Accordingly, it is still another object of the embodiment of the present invention to simplify the frequency components by varying the sequence in which data items are selected for each horizontal period without fixing the sequence.
The present invention has its aspects related to the following forms:
(Liquid Crystal Driver)
In accordance with the first aspect of one embodiment of the present invention, a liquid crystal drive unit has a scanning electrode driver, a signal electrode driver, a frame memory incorporated in the signal electrode driver for storing display data, and a gray-scale display unit incorporated therein. The liquid crystal drive unit controls display according to the multi-line selection driving method. The gray-scale display unit achieves gray-scale display using n-bit data (where n denotes a natural number, or preferably a natural number ranging from 1 to 4, or more preferably 2 or 3). One horizontal period is divided into n selection periods whose temporal widths are weighted in association with display data.
The signal electrode driver has a controller for varying the sequence in which the plurality of differently weighted selection periods is selected for each horizontal period.
In the liquid crystal drive unit according to the embodiment of the present invention, the sequence in which the plurality of differently weighted selection periods is selected may be differentiated between a first liquid crystal electrode and a second liquid crystal electrode which are mutually adjoining in the signal electrode driver.
In another embodiment of the present invention, the sequence in which the plurality of differently weighted selection periods is selected may be identical between one horizontal period within which a liquid crystal alternating signal changes and another horizontal period immediately succeeding the one horizontal period.
Moreover, in the liquid crystal drive unit according to another embodiment of the present invention, the temporal widths of the weighted selection periods are preferably variable.
(Liquid Crystal Driving Method)
According to the second aspects of the embodiment of the present invention, there is provided a liquid crystal driving method for displaying display data using a liquid crystal according to the multi-line selection driving method. In one embodiment, assuming that gray-scale display is achieved based on n-bit data (where n denotes a natural number), one horizontal period is divided into n selection periods whose temporal widths are weighted differently according to the display data. The sequence in which the plurality of differently weighted selection periods is selected is varied for each horizontal period.
In the liquid crystal driving method according to the embodiment of the present invention, the timing when a liquid crystal driving voltage makes a transition is preferably changed between successive outputs of a signal electrode driver.
In the liquid crystal driving method according to the embodiment of the present invention, the timing when the liquid crystal driving voltage makes a transition is preferably not fixed but varied for each horizontal period.
Furthermore, in the liquid crystal driving method according to the embodiment of the present invention, the liquid crystal driving voltage is preferably caused to make a transition when one horizontal period within which a liquid crystal alternating signal changes shifts to an immediately succeeding horizontal period.
Furthermore, in the liquid crystal driving method according to embodiment of the present invention, the timing when the liquid crystal driving voltage makes a transition may preferably be identical between the horizontal period within which the liquid crystal alternating signal changes and a horizontal period immediately succeeding the horizontal period.
(Liquid Crystal Display Device)
The present invention also relates to a liquid crystal display device having the liquid crystal drive unit provided according to the first aspect of the embodiment of the present invention.
The liquid crystal display device in accordance with the embodiment of the present invention can successfully achieve gray-scale display without deterioration in contrast and occurrence of flickers or crosstalk while resolving the drawbacks that underlie the conventional PWM system and realizing low power consumption.
(Electronic Apparatus Having the Liquid Crystal Display Device)
The liquid crystal display device in accordance with the embodiment of the present invention is preferably adapted to electronic apparatuses including information apparatuses such as PDAs and portable telephones, portable game machines, and home electric appliances.